1. Field of the Invention
The present invention relates to an aqueous ink particularly suitable for use in an ink jet recording method, and an ink set, an image forming method and an image forming apparatus using such an aqueous ink.
2. Description of the Related Art
An ink jet recording method is widely used in various fields because of the features that recording can be conducted with a low noise, and high-resolution images can be obtained at high speed by using a recording head having a high-density multi-orifice.
Inks (pigment inks) containing a pigment as a coloring material capable of obtaining recorded articles high in image density and excellent in fastness properties have heretofore been used as inks used in the ink jet recording method. With respect to the pigment inks, black inks containing carbon black and a dispersant and black inks containing a self-dispersion pigment capable of dispersing the pigment without using a dispersant have been proposed (see Japanese Patent Application Laid-Open Nos. S63-152681, S64-006074 and H08-003498).
In recent years, full-color recorded articles have been generally used as various kinds of report materials and notice materials in offices, schools and stores. Therefore, the ink jet recording method is required to form not only a high-quality black image, but also a high-quality full-color image. Upon the formation of such a full-color image, a wide variety of so-called plain papers such as paper for copying, paper for reporting, notepaper, letter paper, bond paper and continuous business forms are used. From the viewpoints of environmental protection and resource protection in recent years, plain paper containing a recycled material has been widely used as a recording medium. Even when a wide variety of plain papers are used as the recording medium, thus, investigations are carried out from various points of view such as compositions and physical properties of inks for forming high-quality images.
In the ink jet recording method, it is widely conducted to use, as a black ink, such a pigment ink as described above in combination with other inks, particularly color inks to form a full-color image. Specific examples of such color inks include at least one selected from a magenta ink, a cyan ink, a yellow ink, a red ink, a green ink and a blue ink. At this time, for a general ink set, it is conducted to adjust the properties of the respective inks as follows. More specifically, with respect to the color inks, it is conducted to make the permeability of the color inks into a recording medium relatively high for the purpose of improving the fixing ability of the inks and inhibiting the occurrence of bleeding at a boundary between images formed with a plurality of color inks. With respect to the black ink, it is conducted to set the permeability of the black ink into the recording medium relatively low compared with the color inks for the purpose of improving character quality. In other words, it is conducted to make the surface tension of the black ink and the surface tension of each color ink relatively high and relatively low, respectively. When an ink set having such black ink and color inks is used to form images, the following problem may be caused in some cases. Namely, a phenomenon (bleeding) that bleeding occurs at a boundary portion between an image formed with the black ink and an image formed with the color ink or a phenomenon that the black ink is unevenly mixed with the color ink to lower image quality may be caused in some cases. In order to inhibit the occurrence of this bleeding, there has been proposed a recording method in which a first ink and a second ink which reacts with the first ink are used and the second ink is applied so as to overlap a region to which the first ink is applied (see Japanese Patent Application Laid-Open No. H11-343441).
In the case where a color ink and a black ink containing a pigment are applied so as to overlap each other for the purpose of inhibiting the occurrence of bleeding, color unevenness in which the image density or color tone of the resulting image varies may occur in some cases according to the order of application of the color ink and the black ink. In order to inhibit the occurrence of this color unevenness, proposals have been made as to such an ink set that a black ink contains a pigment and a salt, and a color ink contains a component for destabilizing the dispersion state of a pigment in the black ink, and to an image forming method (see Japanese Patent Application Laid-Open Nos. 2001-150793 and 2001-152059).
In recent years, for achieving far excellent image quality in the ink jet recording method, it has been conducted to bi-directionally scan a recording head in a main scanning direction 2 times or more, thereby recording an image having the same width as the length of the recording head (hereinafter referred to as “multi-pass recording”). When the multi-pass recording is conducted, the recording time may not be sufficiently shortened in some cases even when the number of nozzles from which one kind of ink is ejected is increased, or when the width along which a plurality of ejection orifices ejecting one kind of ink is arranged is increased. Thus, in order to shorten the recording time, it is attempted to conduct recording in both directions of a forward direction and a backward direction of a main scan of the recording head (hereinafter referred to as “bi-directional recording”), or to record an image of a unit region by one main scan of the recording head (hereinafter referred to as “one-pass recording”). Recording is conducted by combining the one-pass recording with the bi-directional recording, i.e., one-pass bi-directional recording is conducted, whereby there is no need to scan one unit region on a recording medium plural times, so that the recording time can be greatly shortened.
However, as a result of an investigation by the present inventors, it has been found that when recording is conducted at high speed like the one-pass bi-directional recording, such a problem as described below is caused. Such a problem that bleeding is markedly caused has been found. And besides, such a problem that color unevenness is markedly caused when a black ink and a color ink which reacts with the black ink are applied so as to overlap each other like the inventions described in Japanese Patent Application Laid-Open Nos. 2001-150793 and 2001-152059 for the purpose of inhibiting the occurrence of bleeding has been found. This color unevenness is particularly conspicuous in such an image that an image region where recording is conducted in such a manner that a color ink having reactivity overlaps a black ink and an image region where recording is conducted in such a manner that the black ink overlaps the color ink having reactivity coexist. In short, the inventions described in Japanese Patent Application Laid-Open Nos. 2001-150793 and 2001-152059 are insufficient as a method of forming a high-quality image of a high level required in recent years when a wide variety of plain papers are used as recording media.